Wash and trim the rhubarb, then slice it into 2cm pieces. Place the shallots, cider vinegar, ginger and sugar in a pan and season with sea salt and black pepper. Bring to the boil for 5 minutes, then add the rhubarb and red chilli. Reduce the heat and simmer for 20 minutes or until slightly thickened.
If you are making the relish to use at a later date, you can put it in a sterilised jar whilst still hot and store in a cool, dark place.